1. Field of the Invention
The present invention relates to a shaft seal mechanism appropriately used for a rotating shaft or the like of a large size fluid machine, such as a gas turbine, steam turbine, compressor, water turbine, refrigerator, pump or the like and also relates to a shaft seal mechanism assembling structure and large size fluid machine both using this shaft seal mechanism.
2. Description of the Prior Art
Generally, a shaft seal mechanism is provided around a rotating shaft of a gas turbine, steam turbine or the like, for reducing leakage of working fluid leaking to a lower pressure side from a higher pressure side. As one example of such a shaft seal mechanism, a known leaf seal is shown in the Japanese laid-open patent application 2002-13647.
FIG. 9 is a cross sectional view of one example of a prior art leaf seal (shaft seal mechanism) of the kind mentioned above, wherein this leaf seal is seen on a cross section along an axis of a rotating shaft. In FIG. 9, numeral 1 designates a leaf seal and numeral 2 a rotating shaft. The leaf seal 1 is constructed such that a plurality of thin plates 3 of a flat shape having a predetermined size of a plate width in an axial direction of the rotating shaft 2 are arranged in layers in which a minute gap is provided between each of the thin plates 3 in a circumferential direction of the rotating shaft 2 so that a thin plate assembly 9 forms an annular shape. These thin plates 3 have their outer circumferential proximal end side fixed to a split housing or leaf seal ring 5 (5a, 5b) via a brazed portion 4, and their inner circumferential distal end side arranged at an incline with an acute angle relative to an outer circumferential surface of the rotating shaft 2, so as to make a slidable contact with the outer circumferential surface of the rotating shaft 2 by a pre-load. It is to be noted that the leaf seal ring 5 is constructed by assembling together a pair of split seal rings 5a, 5b. Also, each of the thin plates 3, when seen on a plan view, has a T-shape in which the width w1 of the above-mentioned outer circumferential proximal end side is larger than the width w2 of the above-mentioned inner circumferential distal end side.
By the construction mentioned above, the thin plates 3 seal the outer circumferential surface of the rotating shaft 2, and thereby an annular space formed around the rotating shaft 2 is divided into a higher pressure side area and a lower pressure side area. Also, the leaf seal ring 5 comprises a higher pressure side plate 7 on the side of the higher pressure side area, and a lower pressure side plate 8 on the side of the lower pressure side area, so that the thin plates 3 are fitted in between the higher pressure side plate 7 and the lower pressure side plate 8. The respective side plates 7, 8 are also arranged to function as a guide plate for guiding a direction to which pressure acts.
The leaf seal 1 constructed as mentioned above is inserted to be retained in a concave groove 10 of a T-shape formed in a stator side. When the rotating shaft 2 rotates, a dynamic pressure effect is caused by the rotation of the rotating shaft 2, which causes the distal end of each of the thin plates 3 to be levitated away from the outer circumferential surface of the rotating shaft 2 so that there is no contact between the distal ends of the thin plates 3 and the rotating shaft 2. Thereby, abrasion of the thin plates 3 is avoided and the seal life is elongated.
In the prior art shaft seal mechanism (the leaf seal 1), a desired seal performance cannot be stably obtained due to the three shortcomings mentioned below:
(1) For a device in which the shaft seal mechanism (the leaf seal 1) is to be provided, there are strong demands to make the device compact, and efforts are being made for reducing the entire size of the shaft seal mechanism, for example by reducing the thickness. However, as mentioned below, assembly of a smaller shaft seal mechanism into the stator is difficult, which causes problems with the manufacture and employment of the device.
In order to make the shaft seal mechanism smaller, it is considered whether to make the leaf seal ring 5 side thinner. In this case, an optimized shape of the leaf seal ring 5 will have a cross-sectional T-shape having the radial directional portion elongated and the outer circumferential portion formed larger than the inner circumferential portion so as to meet the shape of the thin plates 3. The concave groove to retain the leaf seal ring 5 is also needed to be made in such a shape as to have the radial directional portion deepened and the bottom portion (the outer circumferential portion) formed larger. But to create such a shape of the concave groove in the stator is generally difficult, and even if a compact shaft seal mechanism is developed, there might be a case where actual employment thereof is difficult. Hence, where the shaft seal mechanism, when seen on a cross section including an axis of the rotating shaft, has a shape having the radial directional portion elongated and the outer circumferential proximal end side formed larger than the inner circumferential distal end side, a structure into which this shaft seal mechanism can be easily assembled is desired. It is also desired to reduce the size of the presently employed thin plates 3. But if the thin plates 3 are smaller than the present size, there is a possibility that a desired seal performance may not be stably obtained.
(2) At the turbine start-up time, the leaf seal 1, experiences a downward-acting force caused by its own weight. If an eccentricity is caused by this force, there is a possibility that the distal ends of the annularly arranged thin plates 3 will strongly contact with the outer circumferential surface of the rotating shaft 2 at one place in the circumferential direction (upper portion). If the rotating shaft 2 is rotating while such a strong contact is being maintained, there is a risk that the thin plates 3 and the rotating shaft 2 will be damaged. Therefore, it is considered that a spring member is fixed to the stator side to support the weight of the leaf seal 1 (illustration omitted). If the leaf seal 1 is so levitated, the above-mentioned problems will be avoided.
However, as seen on the cross section of FIG. 9, during continuous operation, the leaf seal 1 receives a fluid force toward the lower pressure side area from the higher pressure side area. However, at the turbine start-up time, the leaf seal 1 receives a fluid force acting in the reverse direction because the pressure in the turbine is reduced to vacuum. Hence, when the start-up state is changed over to the continuous operation state, the leaf seal 1 receives the fluid force reversed from one direction to the other in the rotor axial direction and thereby the leaf seal 1 makes a sliding motion along the rotor axial direction by the length of the fitting allowance relative to the stator side.
If the above-mentioned spring member is fixed to the stator side, when the leaf seal 1 makes the sliding motion, the leaf seal 1 generates such a force as to bend the spring member in the rotor axial direction at the outer circumferential surface portion of the leaf seal 1. The spring member, while receiving such a bending force, may by some chance bite into the outer circumferential surface of the leaf seal 1 so that the normal activating function thereof cannot be exhibited. Then, an eccentric activating force is given onto the leaf seal 1 and this invites a possibility that the seal performance of the leaf seal 1 is badly influenced. Thus, a means is desired which allows a stable seal performance to be obtained, but does not cause any interference with the activating member.
(3) While the prior art leaf seal 1 is manufactured such that each of the thin plates 3 is fitted in between the two split leaf seal rings 5a, 5b, and the jointing portion between the split leaf seal rings 5a, 5b is fixed by welding or bolting, it is known that a gap size formed between the thin plates 3 and the lower pressure side plate 8 affects the seal performance of the leaf seal 1. Hence, it is desired to control this gap size so as to be maintained as designed. However, at the present situation, because of various reasons, such as welding strain caused at the manufacturing time, excess torque of bolting, and working accuracy of the split leaf seal rings 5a, 5b, it is difficult to control the gap size formed between the thin plates 3 and the lower pressure side plate 8 to be maintained as designed. Thus, a means by which a desired seal performance can be stably obtained is desired.